unicode

Functions for converting Unicode characters

This module contains functions for converting between different character representations. Basically it converts between iso-latin-1 characters and Unicode ditto, but it can also convert between different Unicode encodings (like UTF-8, UTF-16 and UTF-32).

The default Unicode encoding in Erlang is in binaries UTF-8, which is also the format in which built in functions and libraries in OTP expect to find binary Unicode data. In lists, Unicode data is encoded as integers, each integer representing one character and encoded simply as the Unicode codepoint for the character.

Other Unicode encodings than integers representing codepoints or UTF-8 in binaries are referred to as "external encodings". The iso-latin-1 encoding is in binaries and lists referred to as latin1-encoding.

It is recommended to only use external encodings for communication with external entities where this is required. When working inside the Erlang/OTP environment, it is recommended to keep binaries in UTF-8 when representing Unicode characters. Latin1 encoding is supported both for backward compatibility and for communication with external entities not supporting Unicode character sets.

This function converts a possibly deep list of integers and
binaries into a list of integers representing unicode
characters. The binaries in the input may have characters
encoded as latin1 (0 - 255, one character per byte), in which
case the Encoding parameter should be given as
latin1, or have characters encoded as UTF-8, in
which case the Encoding should be given as
unicode. Only when the Encoding is unicode,
integers in the list are allowed to be grater than 255.

If Encoding is latin1, the Data parameter
corresponds to the iodata() type, but for unicode,
the Data parameter can contain integers greater than 255
(unicode characters beyond the iso-latin-1 range), which would
make it invalid as iodata().

The purpose of the function is mainly to be able to convert
combinations of unicode characters into a pure unicode
string in list representation for further processing. For
writing the data to an external entity, the reverse function
erlang:characters_to_utf8/2
comes in handy.

If for some reason, the data cannot be converted, either
because of illegal unicode/latin1 characters in the list, or
because of invalid UTF-8 encoding in any binaries, an error
tuple is returned. The error tuple contains the tag
error, a list representing the characters that could be
converted before the error occured and a representation of the
characters including and after the offending integer/bytes. The
last part is mostly for debugging as it still constitutes a
possibly deep and/or mixed list, not necessarily of the same
depth as the original data. The error occurs when traversing the
list and whatever's left to decode is simply returned as is.

However, if the input Data is a pure binary, the third
part of the error tuple is guaranteed to be a binary as
well.

Errors occur for the following reasons:

Integers out of range - If Encoding is
latin1, an error occurs whenever an integer greater
than 255 is found in the lists. If Encoding is
unicode, error occurs whenever an integer greater than
16#10FFFF (the maximum unicode character) or in the
range 16#D800 to 16#DFFF (invalid unicode
range) is found.UTF-8 encoding incorrect - If Encoding is
unicode, the bytes in any binaries have to be valid
UTF-8. Errors can occur for various
reasons, including "pure" decoding errors
(like the upper
bits of the bytes being wrong), the bytes are decoded to a
too large number, the bytes are decoded to a code-point in the
invalid unicode
range or encoding is "overlong", meaning that a
number should have been encoded in fewer bytes. The
case of a truncated UTF-8 is handled specially, see the
paragraph about incomplete binaries below. If
Encoding is latin1, binaries are always valid
as long as they contain whole bytes,
as each byte falls into the valid iso-latin-1 range.

A special type of error is when no actual invalid integers or
bytes are found, but a trailing binary() consists of too
few bytes to decode the last character. This error might occur
if bytes are read from a file in chunks or binaries in other
ways are split on non UTF-8 boundaries. In this case an
incomplete tuple is returned instead of the error
tuple. It consists of the same parts as the error tuple, but
the tag is incomplete instead of error and the
last element is always guaranteed to be a binary consisting of
the first part of a (so far) valid UTF-8 character.

If one UTF-8 characters is split over two consecutive
binaries in the Data, the conversion succeeds. This means
that a character can be decoded from a range of binaries as long
as the whole range is given as input without errors
occuring. Example:

This function behaves as erlang:characters_to_list/2,
but produces an UTF-8 binary instead of a unicode list. Note
that even if Encoding is given as latin1, the
output is UTF-8. The Encoding defines how input is to be
interpreted, not what output is generated. To convert a possibly
deep list of iso-latin-1 characters to a iso-latin-1 binary, use
iolist_to_binary/1.

Errors and exceptions occur as in erlang:characters_to_list/2,
but of course the second element in the error or
incomplete tuple will be a binary() and not a
list().